Temperature Dependence of Stresses and H Desorption in Porous Silicon
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EXPERIMENTS Boron-doped p-type (1-10 a-cm) and p+-type (< 0.005 Q-cm) (100) Si substrates were used in these experiments. These samples were cleaned under microelectronic conditions as reported previously (9). Before anodisation, an ohmic contact was applied to the back of the substrate by evaporating a thin layer of aluminum. This back electrode was removed by etching in HF after PS preparation, and before stress and H effusion measurements. PS was then prepared by electrolytic etching in a 1:1 mixture of 50% HF and ethanol, using current densities of 25 mA/cm 2 for a duration of 5 min. The samples were then rinsed in deionised water and dried in nitrogen before measurement. 555
Mat. Res. Soc. Symp. Proc. Vol. 358 0 1995 Materials Research Society
Stress in the as-prepared PS film was determined using two different methods, which gave similar results: a DEKTAK 3030 stylus profilometer measurement and a laser beam reflection technique. For the latter method, which is suitable for in-situ mechanical stress measurements at high temperature, samples were kept under N2 flow. In these measurements, the radius of curvature of 50 mm-diameter both-sidespolished silicon wafers was determined. The laser-based optical lever technique provides a good combination of accuracy, convenience and speed for most applications (10). We have measured the deformation along two perpendicular axes, and found that it is isotropic in the film plane. We may therefore use the Stoney equation (11) to determine the total stress on the film, at: t
ESi A Si si3 dpsr2
Here, ESi and vSi are respectively the Young's modulus and the Poisson ratio for the Si substrate, dSi and dpS the thicknesses of the substrate and of the porous silicon, and finally, A,the deflection at a distance r ( rmax=1 .8cm ) from the center of the wafer. The composite elastic constant, Ec= Esi is (11) 1.805x10 1 2 dynes/cm 2 (180.5 GPa) 1-VSi
for (100) Si. The net increase in convex or concave substrate curvature compared to that of the substrate without PS correspond to compressive or tensile stress in the PS film. In what follows, compressive stresses are negative and tensile are positive. The heating rate was 20°C/min from room temperature to 9000 C. Since the stress measurement scan requires only a few seconds, we used continuous heating and cooling instead of instead of holding and stabilizing the temperature for each measurement. H effusion measurements were performed in a high vacuum chamber using a quadrupole residual gas analyzer (or mass spectrometer) with the same heating rate (20 0C/min), in order to establish a comparison between these two experiments. RESULTS Stress Measurements The variation of stress as a function of annealing temperature is shown in Fig.l. Asprepared films, measured 5 min. after anodisation, at room temperature, are in a state of moderate compression, with a stress of 6 MPa. In general, stress is the sum of two terms: a thermal component, ath, due to the difference between the thermal expansion coefficients of the film and the substrate,
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